Negative-limited lithium-ion battery

ABSTRACT

A rechargeable lithium-ion battery includes a positive electrode that includes a first current collector and a first active material. The battery also includes an electrolyte and a negative electrode that includes a second current collector and a second active material, where the second active material includes a lithium titanate material. The positive electrode has a first capacity and the negative electrode has a second capacity, the second capacity being less than the first capacity such that the rechargeable lithium-ion battery is negative-limited.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation-in-Part of U.S. patentapplication Ser. No. 10/979,040, filed Oct. 29, 2004. The presentapplication is also a Continuation-in-Part of U.S. patent applicationSer. No. 12/112,979, filed Apr. 20, 2008. The entire disclosures of U.S.patent application Ser. No. 10/979,040 and U.S. patent application Ser.No. 12/112,979 are incorporated by reference herein.

BACKGROUND

The present application relates generally to the field of rechargeablelithium-ion batteries or cells.

Lithium-ion batteries or cells (i.e., rechargeable or “secondary”batteries) include one or more positive electrodes, one or more negativeelectrodes, and an electrolyte provided within a case or housing.Separators made from a porous polymer or other suitable material mayalso be provided intermediate or between the positive and negativeelectrodes to prevent direct contact between adjacent electrodes. Thepositive electrode includes a current collector having an activematerial provided thereon, and the negative electrode includes a currentcollector having an active material provided thereon. The activematerials for the positive and negative electrodes may be provided onone or both sides of the current collectors.

FIG. 1 shows a schematic representation of a portion of a lithium-ionbattery 10 such as that described above. The battery 10 includes apositive electrode 20 that includes a positive current collector 22 anda positive active material 24, a negative electrode 30 that includes anegative current collector 32 and a negative active material 34, anelectrolyte material 40, and a separator (e.g., a polymeric microporousseparator, not shown) provided intermediate or between the positiveelectrode 20 and the negative electrode 30. The electrodes 20, 30 may beprovided as relatively flat or planar plates or may be wrapped or woundin a spiral or other configuration (e.g., an oval configuration). Theelectrode may also be provided in a folded configuration.

During charging and discharging of the battery 10, lithium ions movebetween the positive electrode 20 and the negative electrode 30. Forexample, when the battery 10 is discharged, lithium ions flow from thenegative electrode 30 to the positive electrode 20. In contrast, whenthe battery 10 is charged, lithium ions flow from the positive electrode20 to the negative electrode 30.

Once assembly of the battery is complete, an initial charging operation(referred to as a “formation process”) may be performed. During thisprocess, a stable solid-electrolyte-inter-phase (SEI) layer is formed atthe negative electrode and also possibly at the positive electrode.These SEI layers act to passivate the electrode-electrolyte interfacesas well as to prevent side-reactions thereafter.

One issue associated with conventional lithium-ion batteries relates tothe ability of the batteries to withstand repeated charge cycling thatinvolves discharges to near-zero-volt conditions (so-called “deepdischarge” conditions). This deep discharge cycling may decrease theattainable full charge capacity of the batteries, which is known in theart as capacity fade. For example, a battery that initially is chargedto 2.8 volts (V) may experience capacity fade with repeated deepdischarge cycling such that after 150 cycles, the full charge capacityof the battery is much less than the initial capacity.

One consequence of capacity fade is that the batteries requireincreasingly frequent recharging as the capacity fade progresses, whichmay be relatively inconvenient for the user of the batteries. Forexample, certain implantable medical devices may utilize rechargeablebatteries as a power source, and more frequent charging may beinconvenient for a patient.

Rechargeable lithium-ion batteries are conventionally designed so thatthe capacity of the active material provided on the negative electrodeis equal to or greater than the capacity of the active material providedon the positive electrode. One reason for this design rule is the desireto avoid lithium plating at the negative electrode. In lithium plating,lithium ions that would otherwise be responsible for shuttling betweenthe electrodes during charging and discharging of the battery aredeposited on the negative electrode as lithium metal and, due to reasonssuch as isolation or passivation of the lithium metal, thereafter do notparticipate in the charging and discharge function. This lithium platingleads to a decrease in overall battery capacity.

It would be advantageous to provide a rechargeable lithium-ion batterythat utilizes materials that provide increased resistance to lithiumplating, which may eliminate the need to adhere to design rules such asthose requiring capacity balancing of the electrodes. This in turn mayprovide the opportunity to reduce the amount of certain materials usedin batteries. It would also be advantageous to provide a rechargeablebattery (e.g., a lithium-ion battery) with increased resistance tocapacity fade.

SUMMARY

An exemplary embodiment relates to a rechargeable lithium-ion batterythat includes a positive electrode that includes a first currentcollector and a first active material. The battery also includes anelectrolyte and a negative electrode that includes a second currentcollector and a second active material, where the second active materialincludes a lithium titanate material. The positive electrode has a firstcapacity and the negative electrode has a second capacity, the secondcapacity being less than the first capacity such that the rechargeablelithium-ion battery is negative-limited.

Another exemplary embodiment relates to a rechargeable lithium-ionbattery that includes a positive electrode that includes a first currentcollector and a first active material. The battery also includes anegative electrode that includes a second current collector and a secondactive material. The second active material includes a material thatcycles lithium at a potential of greater than 0.2 volts versus a lithiumreference electrode. The battery further includes an electrolytecomprising ethylene carbonate that is free of molten salt. The positiveelectrode has a first capacity and the negative electrode has a secondcapacity, the second capacity being less than the first capacity. Therechargeable lithium-ion battery exhibits improved resistance tocapacity fade after repeated cycling as compared to batteries havingbalanced electrode capacities.

Another exemplary embodiment relates to a rechargeable lithium-ionbattery that includes a positive electrode and a negative electrode thatincludes an active material that cycles lithium at a potential ofgreater than 0.2 volts versus a lithium reference electrode. The batteryalso includes an electrolyte that is free of molten salt, and has anegative-limited cell balance such that the positive electrode hasgreater capacity than the negative electrode. The rechargeablelithium-ion battery exhibits improved resistance to capacity fade afterrepeated cycling as compared to batteries having balanced electrodecapacities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a portion of a lithium-ion batteryaccording to an exemplary embodiment.

FIG. 2 is a schematic cross-sectional view of a portion of a lithium-ionbattery according to an exemplary embodiment.

FIG. 3 is a graph illustrating the capacity fade as a function of cellbalance for batteries having a variety of cell balances.

FIG. 4 is a graph illustrating the fade rate as a function of cellbalance for the batteries described with respect to FIG. 3.

FIG. 5 is a graph illustrating the initial capacities the batteriesdescribed with respect to FIGURE and the capacities of such batteriesafter 700 charge and discharge cycles over a period of eight months.

FIG. 6 is a graph illustrating the specific capacity of the positiveelectrode as a function of cell balance for the batteries described withrespect to FIG. 3.

FIG. 7 is a graph illustrating the battery voltage and the voltage ofthe positive and negative electrodes versus a lithium referenceelectrode during a battery formation process in which a charging voltageof 2.8 volts is used.

FIG. 8 is a graph illustrating two deep discharge cycles for a batterymanufactured using the formation process according to FIG. 5.

FIG. 9 is a graph illustrating the battery voltage and the voltage ofthe positive and negative electrodes versus a lithium referenceelectrode during a battery formation process in which a charging voltageof 3.4 volts is used.

FIG. 10 is a graph illustrating two deep discharge cycles for a batterymanufactured using the formation process according to FIG. 7.

FIG. 11 is a graph illustrating the resistance to capacity fade for thebatteries produced using the formation processes according to FIGS. 5and 7.

FIG. 12 is a graph illustrating the discharge capacity of battery cellsand the effect of overdischarge cycling of such cells.

FIG. 13 is a schematic view of a system in the form of an implantablemedical device implanted within a body or torso of a patient.

FIG. 14 is a schematic view of another system in the form of animplantable medical device.

DETAILED DESCRIPTION

According to an exemplary embodiment, a rechargeable lithium-ion batteryor cell includes a positive electrode having an active material providedthereon and a negative electrode having an active material providedthereon. The active material provided on the negative electrode includesan active material that cycles lithium at greater than 0.2 volts versusa lithium reference electrode. One example of such a material is lithiumtitanate (e.g., Li₄Ti₅O₁₂). An electrolyte is also provided in thebattery which does not include a molten salt (e.g., ionic liquid)component. The capacity of the negative electrode is less than thecapacity of the positive electrode such that the battery isnegative-limited. The negative-limited battery exhibits improvedresistance to capacity fade as compared to batteries that are balanced(i.e., those having capacity ratios between the negative and positiveelectrodes of 1.0).

FIG. 2 is a schematic cross-sectional view of a portion of a battery orcell 200 according to an exemplary embodiment that includes at least onepositive electrode 210 and at least one negative electrode 220. Thesize, shape, and configuration of the battery may be selected based onthe desired application or other considerations. For example, theelectrodes may be flat plate electrodes, wound electrodes (e.g., in ajellyroll, folded, or other configuration), or folded electrodes (e.g.,Z-fold electrodes). According to other exemplary embodiments, thebattery may be a button cell battery, a thin film solid state battery,or another type of lithium-ion battery.

According to an exemplary embodiment, the battery 200 has a rating ofbetween approximately 1 and 1000 milliampere hours (mAh). According toanother exemplary embodiment, the battery has a rating of betweenapproximately 100 and 400 mAh. According to another exemplaryembodiment, the battery is an approximately 300 mAh battery. Accordingto another exemplary embodiment, the battery is an approximately 75 mAhbattery. According to another exemplary embodiment, the battery is anapproximately 10 mAh battery.

The battery case or housing (not shown) is formed of a metal or metalalloy such as aluminum or alloys thereof, titanium or alloys thereof,stainless steel, or other suitable materials. According to anotherexemplary embodiment, the battery case may be made of a plastic materialor a plastic-foil laminate material (e.g., an aluminum foil providedintermediate a polyolefin layer and a nylon or polyester layer).

An electrolyte is provided intermediate or between the positive andnegative electrodes to provide a medium through which lithium ions maytravel. According to an exemplary embodiment, the electrolyte may be aliquid (e.g., a lithium salt dissolved in one or more non-aqueoussolvents). According to an exemplary embodiment, the electrolyte may bea mixture of ethylene carbonate (EC), ethylmethyl carbonate (EMC) and a1.0 M salt of LiPF₆. According to another exemplary embodiment, anelectrolyte may be used that uses constituents that may commonly be usedin lithium batteries (e.g., propylene carbonate, dimethyl carbonate,vinylene carbonate, lithium bis-oxalatoborate salt (sometimes referredto as LiBOB), etc.). It should be noted that according to an exemplaryembodiment, the electrolyte does not include a molten salt.

Various other electrolytes may be used according to other exemplaryembodiments. According to an exemplary embodiment, the electrolyte maybe a lithium salt dissolved in a polymeric material such aspoly(ethylene oxide) or silicone. According to another exemplaryembodiment, the electrolyte may be an ionic liquid such asN-methyl-N-alkylpyrrolidinium bis(trifluoromethanesulfonyl)imide salts.According to another exemplary embodiment, the electrolyte may be a 3:7mixture of ethylene carbonate to ethylmethyl carbonate (EC:EMC) in a 1.0M salt of LiPF₆. According to another exemplary embodiment, theelectrolyte may include a polypropylene carbonate solvent and a lithiumbis-oxalatoborate salt. According to other exemplary embodiments, theelectrolyte may comprise one or more of a PVDF copolymer, aPVDF-polyimide material, and organosilicon polymer, a thermalpolymerization gel, a radiation cured acrylate, a particulate withpolymer gel, an inorganic gel polymer electrolyte, an inorganicgel-polymer electrolyte, a PVDF gel, polyethylene oxide (PEO), a glassceramic electrolyte, phosphate glasses, lithium conducting glasses, andlithium conducting ceramics, among others.

A separator 250 is provided intermediate or between the positiveelectrode 210 and the negative electrode 220. According to an exemplaryembodiment, the separator 250 is a polymeric material such as apolypropylene/polyethelene copolymer or another polyolefin multilayerlaminate that includes micropores formed therein to allow electrolyteand lithium ions to flow from one side of the separator to the other.The thickness of the separator 250 is between approximately 10micrometers (μm) and 50 μm according to an exemplary embodiment.According to a particular exemplary embodiment, the thickness of theseparator is approximately 25 μm, and the average pore size of theseparator is between approximately 0.02 μm and 0.1 μm.

The positive electrode 210 includes a current collector 212 made of aconductive material such as a metal. According to an exemplaryembodiment, the current collector 212 comprises aluminum or an aluminumalloy.

According to an exemplary embodiment, the thickness of the currentcollector 212 is between approximately 5 μm and 75 μm. According to aparticular exemplary embodiment, the thickness of the current collector212 is approximately 20 μm. It should also be noted that while thepositive current collector 212 has been illustrated and described asbeing a thin foil material, the positive current collector may have anyof a variety of other configurations according to various exemplaryembodiments. For example, the positive current collector may be a gridsuch as a mesh grid, an expanded metal grid, a photochemically etchedgrid, or the like.

The current collector 212 has a layer of active material 216 providedthereon (e.g., coated on the current collector). While FIG. 2 shows thatthe active material 216 is provided on only one side of the currentcollector 212, it should be understood that a layer of active materialsimilar or identical to that shown as active material 216 may beprovided or coated on both sides of the current collector 212.

According to an exemplary embodiment, the active material 216 is amaterial or compound that includes lithium. The lithium included in theactive material 216 may be doped and undoped during discharging andcharging of the battery, respectively. According to an exemplaryembodiment, the active material 216 is lithium cobalt oxide (LiCoO₂).According to other exemplary embodiments, the active material may beprovided as one or more additional materials. For example, the activematerial may be LiMn₂O₄ or a material having the formulaLiCo_(x)Ni_((1−x))O₂, where x is between approximately 0.05 and 0.8.According to another exemplary embodiment, the active material is amaterial of the form LiNi_(x)Co_(y)Mn_((1−x−y))O₂ (e.g.,LiNi_(1/3)Co_(1/3)Mn_(1/3)O₂ or LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂). Accordingto another exemplary embodiment, the active material 216 is ametal-doped variety of one of the aforementioned materials, such as amaterial of the form LiM_(x)Co_(y)Ni_((1−x−y))O₂, where M is aluminum ortitanium and x is between approximately 0.05 and 0.3 and y is betweenapproximately 0.1 and 0.3.

For certain applications, it may be desirable to provide a batteryhaving a cell voltage of greater than approximately 3 volts. In suchcases, a higher-voltage active material may be utilized on the positivecurrent collector, such as a material in the formLi_(2−x)Co_(y)Fe_(z)Mn_(4−(y+z))O₈ (e.g.,Li₂Cu_(0.4)Fe_(0.4)Mn_(3.2)O₈). It is believed that such an activematerial may charge up to 5.2 volts versus a lithium referenceelectrode, making it possible to obtain an overall cell voltage of up toapproximately 3.7 volts. Other relatively high-voltage active materialsthat may be used for the positive electrode include LiCoPO₄; LiNiPO₄;Li₂CoPO₄F; Li[Ni_(0.2)Li_(0.2)Mn_(0.6)]O₂; and LiCo_(x)Mn_(2−x)O₄ (e.g.,LiCo_(0.3)Mn_(1.7)O₄). Other high voltage active materials that may beused could be based on the compositions LiM_(x)Mn_(2−x)O₄ (where M=Ni,Fe with 0<x<0.5) or a material having the formulaLi_(1−x/3)M_(0.5−x/2)Mn_(1.5+x/6)O₄ where x=0 to 0.5 and M=Ni, Fe).

According to various other exemplary embodiments, the active materialmay include a material such as a material of the form Li_(1−x)MO₂ whereM is a metal (e.g., LiCoO₂, LiNiO₂, and LiMnO₂), a material of the formLi_(1−W)(M′_(x)M″_(y))O₂ where M′ and M″ are different metals (e.g.,Li(Cr_(x)Mn_(1−x))O₂, Li(Al_(x)Mn_(1−x))O₂, Li(Co_(x)M_(1−x))O₂ where Mis a metal, Li(Co_(x)Ni_(1−x))O₂, and Li(Co_(x)Fe_(1−x))O₂)), a materialof the form Li_(1−w)(Mn_(x)Ni_(y)Co_(z))O₂ (e.g.,Li(Mn_(1/3)Ni_(1/3)Co_(1/3))O₂, Li(Mn_(1/3)Ni_(1/3)Co_(1/3−x)Mg_(x))O₂,Li(Mn_(0.4)Ni_(0.4)Cu_(0.2))O₂, and Li(Mn_(0.1)Ni_(0.1)Co_(0.8))O₂), amaterial of the form Li_(1−W)(Mn_(x)Ni_(x)Co_(1−2x))O₂, a material ofthe form Li_(1−w)(Mn_(x)Ni_(y)Co_(z)Al_(w))O₂, a material of the formLi_(1−w)(Ni_(x)Co_(y)Al_(z))O₂ (e.g., Li(Ni_(0.8)Cu_(0.15)Al_(0.05))O₂),a material of the form Li_(1−w)(Ni_(x)Co_(y)M_(z))O₂ where M is a metal,a material of the form Li_(1−W)(Ni_(x)Mn_(y)M_(z))O₂ where M is a metal,a material of the form Li(Ni_(x−y)Mn_(y)Cr_(2−x))O₄, LiMn₂O₄, a materialof the form LiM′M″₂O₄ where M′ and M″ are different metals (e.g.,LiMn_(2−y−z)Ni_(y)O₄, Li_(z)O₄, LiNiCuO₄, LiMn_(1−x)Al_(x)O₄,LiNi_(0.5)Ti_(0.5)O₄, and Li_(1.05)Al_(0.1)Mn_(1.85)O_(4−z)F_(z)),Li₂MnO₃, a material of the form Li_(x)V_(y)O_(z) (e.g., LiV₃O₈, LiV₂O₅,and LiV₆O₁₃), a material of the form LiMPO₄ where M is a metal orLiM_(x)′M″_(1−x)PO₄ where M′ and M″ are different metals (e.g., LiFePO₄,LiFe_(x)M_(1−x)PO₄ where M is a metal, LiVOPO₄, and Li₃V₂(PO₄)₃, andLiMPO_(4x) where M is a metal such as iron or vanadium and X is ahalogen such as fluorine, and combinations thereof.

A binder material may also be utilized in conjunction with the layer ofactive material 216 to bond or hold the various electrode componentstogether. For example, according to an exemplary embodiment, the layerof active material may include a conductive additive such as carbonblack and a binder such as polyvinylidine fluoride (PVDF) or anelastomeric polymer.

According to an exemplary embodiment, the thickness of the layer ofactive material 216 is between approximately 0.1 μm and 3 mm. Accordingto another exemplary embodiment, the thickness of the layer of activematerial 216 is between approximately 25 μm and 300 μm. According to aparticular exemplary embodiment, the thickness of the layer of activematerial 216 is approximately 75 μm.

The negative electrode 220 includes a current collector 222 that is madeof a conductive material such as a metal. According to an exemplaryembodiment, the current collector 222 is aluminum or an aluminum alloy.One advantageous feature of utilizing an aluminum or aluminum alloycurrent collector is that such a material is relatively inexpensive andmay be relatively easily formed into a current collector. Otheradvantageous features of using aluminum or an aluminum alloy includesthe fact that such materials may have a relatively low density, arerelatively highly conductive, are readily weldable, and are generallycommercially available. According to another exemplary embodiment, thecurrent collector 222 is titanium or a titanium alloy. According toanother exemplary embodiment, the current collector 222 is silver or asilver alloy.

While the negative current collector 222 has been illustrated anddescribed as being a thin foil material, the negative current collectormay have any of a variety of other configurations according to variousexemplary embodiments. For example, the positive current collector maybe a grid such as a mesh grid, an expanded metal grid, a photochemicallyetched grid, a metallized polymer film, or the like.

According to an exemplary embodiment, the thickness of the currentcollector 222 is between approximately 100 nm and 100 μm. According toanother exemplary embodiment, the thickness of the current collector 222is between approximately 5 μm and 25 μm. According to a particularexemplary embodiment, the thickness of the current collector 222 isapproximately 10 μm.

The negative current collector 222 has an active material 224 providedthereon. While FIG. 2 shows that the active material 224 is provided ononly one side of the current collector 222, it should be understood thata layer of active material similar or identical to that shown may beprovided or coated on both sides of the current collector 222.

According to an exemplary embodiment, the negative active material isselected such that it has an average potential that is greater or equalto approximately 0.2 V versus Li/Li⁺ (e.g., according to one particularexemplary embodiment, the negative active material has an averagepotential that is greater or equal to approximately 0.3 V versus Li/Li⁺;according to a particularly preferred embodiment, the negative activematerial is a titanate material having an average potential that isgreater or equal to approximately 1.5 V versus Li/Li⁺). The inventorshave unexpectedly discovered that the use of negative electrodematerials that possess a relatively high average potential versus Li/Li⁺reduces the likelihood of lithium plating. According to one exemplaryembodiment, such a negative active material is used in conjunction witha positive active material that has an average potential of greater thanapproximately 3 V versus Li/Li⁺ (e.g., LiCoO₂).

According to an exemplary embodiment, the negative active material 224is a lithium titanate material such as Li₄Ti₅O₁₂ (sometimes referred toas Li_(1+x)[Li_(1/3)Ti_(5/3)]O₄, with 0≦x<1). Other lithium titanatematerials which may be suitable for use as the negative active materialmay include one or more of the following lithium titanate spinelmaterials: H_(x)Li_(y−x)TiO_(x)O₄, H_(x)Li_(y−x)TiO_(x)O₄,Li₄M_(x)Ti_(5−x)O₁₂, Li_(x)Ti_(y)O₄, Li_(x)Ti_(y)O₄,Li₄[Ti_(1.67)Li_(0.33−y)M_(y)]O₄, Li₂TiO₃, Li₄Ti_(4.75)V_(0.25)O₁₂,Li₄Ti_(4.75)Fe_(0.25)O_(11.88), Li₄Ti_(4.5)Mn_(0.5)O₁₂, and LiM′M″XO₄(where M′ is a metal such as nickel, cobalt, iron, manganese, vanadium,copper, chromium, molybdenum, niobium, or combinations thereof, M″ is anoptional three valent non-transition metal, and X is zirconium,titanium, or a combination of these two). Note that such lithiumtitanate spinel materials may be used in any state of lithiation (e.g.,Li_(4+x)Ti₅O₁₂, where 0≦x≦3).

According to an exemplary embodiment, the lithium titanate may beprovided such that at least five percent is in the form of lithiumtitanate nanoparticles (e.g., having a particle size of less thanapproximately 500 nanometers). The use of such nonoparticles is intendedto provide greater surface area for doping and undoping of lithium ions.

According to other exemplary embodiments, a lithium vanadate (e.g.,Li_(1.1)V_(0.9)O₂) material may be used as the negative active material.Another exemplary embodiment may have the compound TiO₂ (B) with acharge discharge voltage of 2.0-1.0 V versus a lithium referenceelectrode as the negative active material. Other materials havingcycling potentials that exceed that of lithium by several hundredmillivolts and which may be suitable for use as the negative activematerial include the materials listed in Table 1. Such materials may beused alone or in combination with the lithium titanates described aboveand/or any of the other materials listed in Table 1.

TABLE 1 Cycling Potentials (vs Li) Class Compound Vmin Vmax Vavg OxidesTiO₂ (B) 0.8 2.0 1.45 Oxides TiO₂ (Anatase) 1.4 2 1.80 Oxides WO₂ 0.61.3 0.80 Oxides WO₃ 0.5 2.6 1.0 Oxides MoO₂ 1.3 2 1.60 Oxides Nb₂O₅ 1.02 1.50 Oxides LiWO₂ 0.75 Oxides Li_(x)MoO₂ 0.8 2 1.60 Oxides V₆O₁₃ 2.30Oxides Li₆Fe₂O₃ 0.75 Oxides LiFeO₂ 1.0 3.0 2.0 Oxides Fe₂O₃ 0.2 2.0 0.75Oxides MO where M = Co, Ni, Cu or Fe 0.8-1.5 Sulfides FeS₂ 1.3 1.9 1.65Sulfides MoS₂ 1.75 Sulfides TiS₂ 2.00 Alloys Sn—Bi 0.75 Alloys Alloyscomprising of Al, 0.30 Si or Sn and other elements Alloys Sn—Co—C 0.30Alloys Sb 0.90 Alloys NbSe₃ 1.95 Alloys Bi 0.80 Alloys In 0.60 AlloysLixAl 0.36 Alloys LixSn 0 1.3 0.50 Alloys Sn—Sb 0.0-1.0 PolymersPoly(phenylquinoline) 1.50 Polymers Polyparaphenylene 0.70 PolymersPolyacetylene 1.00 Vanadates Li_(x)MVO₄ where M = Ni, 2.0-0.5 Co, Cd, Zn

A binder material may also be utilized in conjunction with the layer ofactive material 224. For example, according to an exemplary embodiment,the layer of active material may include a binder such as polyvinylidinefluoride (PVDF) or an elastomeric polymer. The active material 224 mayalso include a conductive material such as carbon (e.g., carbon black)at weight loadings of between zero and ten percent to provide increasedelectronic conductivity.

According to various exemplary embodiments, the thickness of the activematerial 224 is between approximately 0.1 μm and 3 mm. According toother exemplary embodiments, the thickness of the active material 224may be between approximately 25 μm and 300 μm. According to anotherexemplary embodiment, the thickness of the active material 224 may bebetween approximately 20 μm and 90 μm, and according to a particularexemplary embodiment, approximately 75 μm.

Lithium plating occurs when the potential of the negative electrodeversus a lithium reference electrode reaches 0 volts, and is awell-known phenomenon that can lead to loss in performance oflithium-ion batteries. When used in a negative electrode of alithium-ion battery, lithium titanate active materials cycle lithium ata potential plateau of about 1.55 volts (which is substantially higherthan graphitic carbon, which cycles lithium at approximately 0.1 voltsin the fully charged state). As a result, batteries using lithiumtitanate as a negative active material are less susceptible to lithiumplating than those using carbon-based materials for the negative activematerial.

One advantageous feature of utilizing a negative electrode activematerial such as a lithium titanate material or another material havingan average potential that is greater or equal to approximately 0.2 Vversus Li/Li⁺ is that more favorable design rules may be possible forrechargeable (i.e., secondary) lithium-ion batteries. One such designrule relates to cell balance parameters.

Cell balance refers to the ratio of the negative electrode capacity tothe positive electrode capacity. Thus, the cell balance γ (gamma) can beexpressed as an equation having the form:

$\gamma = \frac{Q_{neg}}{Q_{pos}}$

where Q_(neg) is the capacity of the negative electrode and is equal tothe product of the amount of negative electrode material, the percentageof the active material contributing to the capacity, and the specificcapacity of the active material and Q_(pos) the capacity of the positiveelectrode and is equal to the product of the amount of positivematerial, the percent of the active material contributing to thecapacity, and the specific capacity of active material. According to anexemplary embodiment in which a lithium titanate negative activematerial and a lithium cobalt oxide positive active material are used,the specific capacity of the negative active material may be 165 mAh/g(at C/3-C/1, between 1.2 and 2.0 volts versus Li) and the specificcapacity of the positive active material may be 150 mAh/g (at C/3-C/1,between 4.25 and 3.0 volts versus Li).

Cell balance dictates the mass (deposition) ratio of the two electrodes.In conjunction with charge cutoff voltage, cell balance determines whatfraction of the active Li sites in the two electrode materials isutilized during charge and discharge. For example, in a negative-limiteddesign, nearly all active Li sites in the negative material are utilizedin the negative electrode, but only a fraction of the active sites inthe positive electrode are utilized. The chosen cell balance hencedetermines the specific capacity delivered by both electrodes, and thestability of their performance for repeated cycling. Primaryconsiderations for choosing the cell balance include safety, deliveredcapacity, and capacity fade under regular cycling and deep dischargeconditions.

Rechargeable lithium-ion batteries using carbon-based negative activematerials are typically fabricated such that the capacity of theelectrodes is balanced (i.e., the capacities of the positive electrodeand negative electrode are equal) or positive-limited (i.e., thecapacity of the positive electrode is less than that of the negativeelectrode). The reason for this design rule is that if the battery wasnegative-limited, the voltage of the negative electrode versus a lithiumreference electrode would drop to near zero volts during charging of thebattery, which may result in lithium plating (since carbonaceousnegative electrodes typically operate very close to the potential ofmetallic lithium (0.1-0.2 volts versus a lithium reference electrode), afurther decrease in potential at the negative electrode would result inplating of lithium).

According to an exemplary embodiment in which a battery utilizes anegative electrode active material such as a lithium titanate materialor another material having an average potential that is greater or equalto approximately 0.2 volts versus a lithium reference electrode,rechargeable lithium-ion batteries may be fabricated with anegative-limited design in which the capacity of the negative electrodeis less than that of the positive electrode. For example, for a lithiumtitanate active material having a potential plateau of approximately1.55 volts, a cell balance of between approximately 0.80 to 0.93 may beused (i.e., the nominal capacity of the negative electrode may bebetween 80 and 93 percent of the nominal capacity of the positiveelectrode; stated another way, the ratio of the positive electrodecapacity to the negative electrode capacity is between approximately1.07 and 1.25). According to another exemplary embodiment, the cellbalance may be less than approximately 0.8 (e.g., 0.73) (that is, theratio of the positive electrode capacity to the negative electrodecapacity may be greater than 1.25). According to another exemplaryembodiment, the cell balance may be approximately 0.85 (ratio of thepositive electrode capacity to the negative electrode capacity ofapproximately 1.176). According to another exemplary embodiment, thecell balance may be approximately 0.84 (ratio of the positive electrodecapacity to the negative electrode capacity of approximately 1.19).According to another exemplary embodiment, the cell balance may beapproximately 0.93 (ratio of the positive electrode capacity to thenegative electrode capacity of approximately 1.075). According toanother exemplary embodiment, the cell balance may be less thanapproximately 0.90 (ratio of the positive electrode capacity to thenegative electrode capacity of approximately 1.11).

The inventors have unexpectedly discovered that negative-limitedrechargeable lithium-ion batteries (where the negative electrode activematerial is a lithium titanate material or another material having anaverage potential that is greater or equal to approximately 0.2 voltsversus a lithium reference electrode) exhibit lower capacity fade andlower power fade due to reduced likelihood of over-charging of thepositive electrode.

FIG. 3 is a graph illustrating the capacity fade behavior for batterieshaving cell balances of 0.73, 0.78, 0.84, 0.88, 0.93, 0.98, and 1.02(all batteries utilized LiCoO₂ as a positive active material andLi₄Ti₅O₁₂ as a negative active material). The batteries were subjectedto 700 charge and discharge cycles over the course of an eight monthperiod, and the discharge capacity of each of the batteries was measuredafter each cycle. For clarity, FIG. 3 illustrates the dischargecapacities of each of the cells only for every 100^(th) cycle, and isexpressed as a percentage of the original discharge capacities for thecells (e.g., a new battery would have 100% of its capacity, and as thebattery is cycled, its capacity is some lower percentage of the originaldischarge capacity).

As illustrated in FIGS. 3 and 4, batteries that had lower cell balancesretained a greater amount of their original discharge capacity withrepeated charge and discharge cycling. Stated another way, the greaterthe cell imbalance (i.e., the more negative-limited the batteries were),the better the batteries resisted capacity fade with increased cycling.For example, as shown in FIG. 4, the percentage of capacity fade percycle was significantly less for the battery having a 0.73 cell balanceas compared to the battery having a 0.98 cell balance. It should benoted that although FIG. 3 appears to show a capacity increase above100% for the 0.93 cell balance batteries, this is a result of anexperimental artifact and should not be interpreted to mean that thedischarge capacity of this battery increased above 100% of its originaldischarge capacity.

One consequence of fabricating negative-limited batteries of the typedescribed above is that such batteries initially have lower overalldischarge capacities than balanced batteries (i.e., batteries having acell balance of 1.0). For example, FIG. 5 is a graph illustrating therelationship between cell balance and the initial overall batterycapacities and the “faded” capacities (i.e., the capacities after thebatteries were subjected to 700 charge and discharge cycles over thecourse of eight months) for batteries having cell balances of 0.73,0.78, 0.84, 0.88, 0.93, 0.98, and 1.02. The capacities are shown aspercentages of the capacity that a balanced cell would be expected toexhibit. As shown in FIG. 5, increasing the cell imbalance (i.e., makingbatteries more negative-limited) results in lower initial capacity.

The inventors have unexpectedly found, however, that althoughnegative-limited batteries may have lower initial capacities thanbalanced cells, the more negative-limited batteries may actually havehigher capacities than balanced batteries after repeated cycling due totheir improved resistance to capacity fade. As shown in FIG. 5, after700 charge/discharge cycles over the course of eight months, thebatteries having cell balances of 0.98 and 1.02 actually have lowerdischarge capacities than the batteries having cell balances of 0.73,0.78, 0.84, 0.88 and 0.93. As a result, rechargeable lithium-ionbatteries may be fabricated with a negative-limited cell imbalance thatrequires less negative active material without sacrificing long-termperformance of the batteries.

FIG. 6 illustrates the specific capacity of the positive electrode as afunction of cell balance for the batteries having cell balances of 0.73,0.78, 0.84, 0.88, 0.93, 0.98, and 1.02. This shows that with lower cellbalance, a lower specific capacity of the positive is obtained. In otherwords, with a lower cell balance, the positive electrode is lessstressed (worked), which leads to lower capacity fade. It should benoted that the data shown in FIG. 6 correlates well with the data shownin FIG. 4, which emphasizes the link between positive specific capacityand capacity fade.

As a battery ages, both the negative electrode and the positiveelectrode fade in capacity. The rate at which the negative and positiveelectrodes fade individually will affect the overall cell balance. Forexample, if the positive electrode fades at a greater rate than thenegative electrode, the battery will shift toward being lessnegative-limited (i.e., the cell balance will increase with aging), andif the negative electrode fades at a greater rate than the positiveelectrode, the battery will shift toward being more negative-limited(i.e., the cell balance will decrease with aging). It is thus possiblethat a battery that starts off as being negative-limited may eventuallybecome positive-limited if the positive electrode fades at a greaterrate than the negative electrode (and a positive-limited battery maybecome negative-limited if the negative electrode fade rate exceeds thatof the positive electrode fade rate).

It should be noted, however, that the capacity of the battery, and thusthe overall fade rate for the battery, will be determined by thelimiting electrode. That is, the overall fade rate for anegative-limited battery will be determined by the fade rate of thenegative electrode. Not until the battery goes from beingnegative-limited to positive-limited will the fade rate of the batterybe driven by the positive electrode fade rate.

By way of example, a battery having a positive electrode capacity of 100mAh and a negative electrode capacity of 90 mAh will have an overallcell capacity of 90 mAh. After a period of time, the positive electrodecapacity may decrease to 95 mAh while the negative electrode capacitymay decrease to 89 mAh, in which case the overall cell capacity will be89 mAh. Here the positive electrode would have faded five times morethan the negative electrode, but because the battery fade is limited bythe negative electrode, the overall battery fade was one-fifth what itwould have been if the positive electrode determined the capacity of thebattery. In contrast, if this battery continues to age to a point whereit is positive-limited, then the capacity fade of the battery will bedriven by the positive electrode. For example, if the battery ages tothe point where the negative electrode capacity is 87 mAh (a decrease of3 mAh from the original 90 mAh) and the positive electrode capacitydrops to 80 mAh (a decrease of 20 mAh), the battery will bepositive-limited. If at a later time the negative electrode capacitydrops to 86 mAh and the positive electrode capacity drops to 75 mAh, thecapacity of the battery will be 75 mAh. That is, the quicker-fadingelectrode now determines the capacity of the battery such that theslower-fading electrode will not constrain the overall capacity fade forthe battery.

To reduce the overall capacity fade rate for a battery, then, it isdesirable to limit the battery by the slowest-fading electrode and toprevent the cell balance from switching during the useful life of thebattery (or to at least significantly delay the switch from occurring).One further consideration to take into account is the fact that it isexpected that the fade rate of the quicker-fading electrodes willactually accelerate as the cell approaches a balanced condition. Thissuggests that there is a compound benefit to increasing the cellimbalance to limit the cell to the slower-fading electrode (e.g., makinga cell more negative-limited where the negative electrode fades moreslowly than the positive electrode), since there will be a non-linearrelationship between the initial cell balance and the time (or number ofcharge-discharge cycles) it takes for the battery to switch over tobeing limited by the quicker-accelerating electrode.

According to various exemplary embodiments described herein that usenegative electrode active materials that cycle lithium at a voltagegreater than approximately 0.2 V versus a lithium reference electrode(for example, a lithium titanate material such as Li₄Ti₅O₁₂), the faderate of the positive electrode is greater than that of the negativeelectrode. Accordingly, it is advantageous to provide a battery that isnegative-limited, and the more negative-limited the battery is, thebetter it is expected will be the resistance to overall battery capacityfade. Indeed, this is consistent with the results shown in FIG. 2.Reducing the fade rate for these batteries is particularly beneficialfor applications with very long product life cycles and where goodforecast of capacity fade is essential (such as implanted medicaldevice).

Another advantageous feature of negative-limited batteries such as thosedescribed herein is that such batteries do not require electrolyteadditives that are intended to prevent degradation due to oxidation ofthe positive electrode. For example, in balanced batteries or batterieshaving excess positive electrode capacity, additives such as biphenylare included in the electrolyte. Besides the obvious disadvantage thatthe inclusion of such additives increases the cost of the batteries, theadditives also complicate the electrolyte chemistry and typically leadto gas formation and swelling when oxidized. Eliminating the additivesmay thus reduce the cost of producing the battery, simplify theelectrolyte chemistry, and reduce the amount of gas formation andbattery swelling that may occur with repeated cycling of the battery.

Yet another advantageous feature of the use of negative-limited designsis that higher energy densities may be achieved as compared to balancedcells. Negative active materials such as lithium titanate have lowerenergy densities than positive active materials such as LiCoO₂ (i.e.,for equal volumes of Li₄Ti₅O₁₂ and LiCoO₂, the LiCoO₂ will have moreactive lithium that contributes to the battery capacity). As a result,altering the cell balance to a negative-limited design may have asubstantial effect on the volume required for the negative activematerial for a given amount of positive active material. This may allowfor the production of smaller batteries for the same amount of positiveactive material. These smaller batteries may also exhibit improvedenergy density as compared to larger balanced cells. For example, asdescribed with respect to FIG. 4, negative-limited cells would beexpected to have higher capacities than balanced cells after repeatedcycling (e.g., for 500 charge/discharge cycles); smallernegative-limited batteries having higher capacities would have a higherenergy density than the larger balanced cells.

Negative-limited cells may also provide cost savings as compared tobalanced cells. As previously described, certain electrolyte additivesmay be omitted from the negative-limited cells. Other materials savingsmay be realized as a result of using less negative active materialand/or producing smaller batteries (e.g., the container may be smaller,which would require less material).

One particular advantage associated with negative-limited batteries(using negative active materials such as a lithium titanate material oranother material having an average potential that is greater or equal toapproximately 0.2 volts versus a lithium reference electrode) isdescribed with respect to FIGS. 5-9. According to an exemplaryembodiment, an improved formation process may be used in conjunctionwith such batteries that provides enhanced tolerance to repeated deepdischarge conditions, as evidenced, for example, by improved resistanceto capacity fade.

During a formation (i.e., initial charging of the battery) process,lithium-ion batteries are charged at a relatively low rate (such as C/10or slower) to the maximum operating voltage of the battery. By way ofexample, for a lithium-ion battery (LiCoO₂ positive active material,lithium titanate negative active material) that is configured to have anintended operating voltage range between approximately 2.8 volts (fullcharge) and 1.8 volts (discharge cut-off voltage), a formation processmight involve charging the battery at a voltage of 2.8 volts at a rateof C/10 and then holding the battery at the 2.8 volt level for fourhours. FIG. 7 illustrates the voltage behavior for a battery undergoingsuch a formation process, and includes a curve 310 representative of theoverall battery voltage, a curve 312 representative of the positiveelectrode potential versus a lithium reference electrode, and a curve314 representative of the negative electrode potential versus a lithiumreference electrode. As shown in FIG. 7, the overall battery voltagegradually increases to a 2.8 volt plateau 311 at the end of formationprocess, while the negative electrode potential versus a lithiumreference electrode drops to a level of between approximately 1.3 and1.4 volts toward the end of curve 314.

The formation process described in the preceding paragraph may result ina relatively low irreversible capacity loss for the battery (e.g.,between approximately 6 and 11 percent) during the formation process.This irreversible capacity loss results primarily from the passivationof either of the two electrodes, and occurs when otherwise cyclablelithium ions in the battery (i.e., lithium ions that shuttle between thepositive and negative electrodes during charging and discharging) failto combine in a reversible way with an electrode active material.

FIG. 8 illustrates the voltage behavior for the battery when it issubjected to two deep discharge cycles where the battery is dischargedto zero volts, and includes a curve 320 representative of the overallbattery voltage, a curve 322 representative of the positive electrodepotential versus a lithium reference electrode, and a curve 324representative of the negative electrode potential versus a lithiumreference electrode. As the overall battery voltage drops toward zerovolts (i.e., a deep discharge condition), the curves 322, 324representing the potentials of the positive and negative electrodesconverge toward and intersect at a point 326 referred to as the zerovolt crossing potential for the battery. As illustrated in FIG. 8, thezero voltage crossing potential for this battery occurs at a point wherethe positive electrode potential versus a lithium reference electrode isless than 2.0 volts (e.g., between approximately 1.6 and 1.8 volts).

FIG. 11 is a graph that includes a curve 330 representative of thedischarge capacity of the battery as it undergoes repeated deepdischarge cycles. The variations in the curves at cycles 50 and 100 area result of two characterization cycles that were run during the test.These characterization cycles consist of (1) a slow rate cycle todetermine trends in full charge/discharge capacity, (2) an applicationrate cycle to determine impact of repeated deep discharge on performancein normal application conditions. Other smaller variations (such asaround cycle 170) are experimental artifacts.

As illustrated by the curve 330 shown in FIG. 11, the discharge capacityof the battery drops as it is subjected to repeated deep dischargecycles. This capacity fade with repeated deep discharge cycles resultsin a battery that must be recharged more frequently, since the batterydoes not hold as much charge as it did after initial formation.

The inventors have determined through experimentation that one factorthat contributes to the capacity fade is that the positive activematerial (in this case LiCoO₂) tends to relatively rapidly degrade whendriven to a potential versus a lithium reference electrode that is lessthan 2.0 volts (e.g., approximately 1.6 volts). It should be noted thatwhere different positive active materials are used, degradation of suchmaterials may occur at a different level. As illustrated in FIG. 8, thepotential of the positive electrode versus a lithium referenceelectrode, as represented by curve 322, drops to a level that is below2.0 volts as the battery is discharged to a near-zero-volt charge stateand the curves 322, 324 approach the zero volt crossing potential 326for the battery.

The inventors have surprisingly discovered that the resistance tocapacity fade for a negative-limited battery that utilizes a lithiumtitanate active material on the negative electrode can be significantlyimproved by using a formation process in which the charging voltage isgreater than the normal fully charged voltage of the battery, with noassociated increase in charging rate (e.g., the same C/10 charging ratemay be used as described in the preceding example).

According to an exemplary embodiment, a lithium-ion battery (LiCoO₂positive active material, lithium titanate negative active material)having an intended operating voltage range from 2.8 volts (fully chargedstate) to 1.8 volts (discharge cut-off voltage) is subjected to aformation process that utilizes a charging voltage of betweenapproximately 3.4 volts and 3.8 volts at a C/10 charge rate, after whichthe battery voltage is held at this level for a period of betweenapproximately 4 and 12 hours. FIG. 9 illustrates the voltage behaviorfor a battery undergoing such a formation process, and includes a curve340 representative of the overall battery voltage, a curve 342representative of the positive electrode potential versus a lithiumreference electrode, and a curve 344 representative of the negativeelectrode potential versus a lithium reference electrode.

As shown in FIG. 9, the battery voltage gradually increases to a 3.4volt plateau 341 at the end of formation process. The higher chargingvoltage during the formation process results in a greater amount ofirreversible capacity loss as compared to formation process using alower charging voltage (e.g., 2.8 volts). For example, the totalirreversible capacity loss obtained using the higher-voltage formationprocess is expected to be between approximately 12 and 20 percent,depending on factors including, for example, the selected cut-offvoltage and hold duration.

The negative electrode potential versus a lithium reference electrodedrops to a level below approximately 0.9 volts (e.g., betweenapproximately 0.5 and 0.8 volts) as the battery reaches its fullycharged state (i.e., the charge top-off voltage during normal use),while the positive electrode potential does not exhibit a large voltageincrease. This is primarily due to the fact that the battery isnegative-limited such that the capacity of the negative electrode isless than that of the positive electrode (i.e., the negative electrodedepletes before the positive electrode, which results in a potentialdrop for the negative electrode).

As the potential of the negative electrode drops below approximately 0.9volts, a reaction takes place in which the solvent component of theelectrolyte (e.g., ethylene carbonate) reduces. In this reductionreaction, a passive film (e.g., lithium carbonate or lithium alkylcarbonate) is formed on the negative electrode. Additionally, anadditive such as carbon or a carbon-based material may be provided inthe negative active material to assist in the formation of the passivefilm and to increase the amount of lithium that reacts irreversibly toform the film (e.g., between approximately 5 and 10 volume percentcarbon may be provided in the lithium titanate material).

One advantageous feature of the reduction reaction is that the increasedirreversible capacity loss of the battery tends to increase the zerovolt crossing potential for the battery, which allows for enhancedresistance to capacity fade since the higher zero volt crossingpotential is above the level where the positive active materialdegrades.

FIG. 10 illustrates the voltage behavior for such a battery when it issubjected to two deep discharge cycles where the battery is dischargedto zero volts, and includes a curve 350 representative of the overallbattery voltage, a curve 352 representative of the positive electrodepotential versus a lithium reference electrode, and a curve 354representative of the negative electrode potential versus a lithiumreference electrode. As the overall battery voltage drops toward zerovolts (i.e., a deep discharge condition), the curves 352, 354representing the potentials of the positive and negative electrodesconverge toward and intersect at the zero volt crossing potential 356for the battery. As illustrated in FIG. 10, the zero voltage crossingpotential for this battery occurs at a point where the positiveelectrode potential versus a lithium reference electrode is greater thanapproximately 3.5 volts, which is above the threshold at which thepositive active material (LiCoO₂) would be expected to degrade. Itshould also be noted that the lithium titanate negative electrode activematerial is also stable at this potential.

The battery produced using the high-voltage formation process exhibitssignificantly enhanced resistance to capacity fade when subjected torepeated deep discharge cycles, as illustrated by the curve 360 in FIG.11. The capacity of the battery is substantially constant overapproximately 170 deep discharge cycles, in contrast to the relativelysignificant capacity fade exhibited by the battery using a moreconventional formation process (curve 330).

Accordingly, the use of a voltage that exceeds the voltage of the fullycharged battery during the formation process acts to induce a largeramount of irreversible capacity loss in the battery than would otherwisebe obtained. This in turn increases the zero voltage crossing potentialfor the battery above the level where the positive active material woulddegrade. As a result, the resistance to capacity fade with repeated deepdischarge cycles is enhanced.

EXAMPLE 1

Lithium ion cells having nominal capacities of approximately 0.15 amperehours (Ah) were prepared having a lithium cobalt oxide (LiCoO₂) activematerial on the positive electrode and a lithium titanate activematerial on the negative electrode. The electrolyte used included amixture of ethylene carbonate, ethylmethyl carbonate, and 1 molar LiPF₆as the lithium salt. The cells were constructed as spirally woundprismatic cells hermetically sealed in a stainless steel can with aglass feedthrough.

The negative and positive electrodes for the cells were prepared via aslurry coating and calendaring process. Both electrodes included theaforementioned active material, a conductive carbon additive, andorganic binder. The mass loading (grams of materials per unit area)during the slurry coating process was controlled on both electrodes toattain a mass ratio of the two active materials to be 0.71. Based on thenominal specific capacities of the two active materials, determined viahalf cell testing (150 mAh/g for lithium cobalt oxide, 165 mAh/g forlithium titanate), the cell balance of this cell was 0.78 (i.e., thenominal capacity of the negative electrode was 78% of the nominalcapacity of the positive electrode).

Cells built with the above design were subjected to a conventionalformation process in which the cells were charged at C/10 (15 mA) to thenormal cut-off voltage of 2.8 volts and were held potentiostatically at2.8 volts for 4 hours. Another group of cells built with the abovedesign were subjected to an improved high voltage formation process. Inthis process, the cells were charged at C/10 (15 mA) to 3.4 volts andwere held potentiostatically at 3.4 volts for 4 hours. After theserespective formation processes, both groups of cells were discharged atC/10 to 1.8 volts and held at 1.8 volts for 4 hours.

The capacities delivered by the cells were measured both during theformation charge process and during the discharge process. The measuredcapacity values are given in Table 2. The formation charge capacity istypically greater than the discharge capacity, which is indicative ofthe irreversible capacity loss accounted for by the irreversibleprocesses. The cells show 6.5% irreversible capacity during theconventional formation process, compared to 12.2% in the improved highvoltage formation process. Thus, the difference in the irreversiblecapacities is approximately 5.7% between the 2.8 volts formation andhigh voltage 3.4 volts formation. This difference is seen to increase asa greater voltage and/or a longer potentiostatic hold duration ischosen. It is noteworthy that this greater irreversible capacity doesnot reduce the reversible capacity of the cell. Cycle 1 dischargecapacity is the same for both groups of cells, as shown in Table 2. Thehigher irreversible capacity is attained by extending the voltage windowbeyond its normal cut-off and thus comes out from the extended capacityand not from the nominal capacity over the voltage range for normalcycling. Thus, unlike typical cases where higher irreversible capacitymeans lower reversible capacity, such a lowering of the reversiblecapacity was not observed.

TABLE 2 Cycle 1 Cycle 1 Cycle 160 Cycle 160 % Fade charge dischargecharge discharge between capacity Capacity capacity capacity cycles 1and (Ah) (Ah) (Ah) (Ah) 160 Cells 0.168 0.157 0.081 0.081 48.4% formedat 2.8 volts Cells 0.180 0.158 0.152 0.152 3.5% formed at 3.4 volts

These cells were then cycled between 2.8 volts to 0.0 volts to determinethe stability of performance for repeated deep discharge to zero volts.During the charge phase of this cycling, the cells were taken to 2.8volts at a 1 C rate (150 mA) and potentiostatically held at 2.8 voltsfor 1 hour. During the discharge phase of the cycling, the cells weretaken to 0.0 volts in a staged manner—first the cells were discharged to1.8 volts at 150 mA, held potentiostatically at 1.8 volts for 1 hour,then discharged to 0.0 volts at 0.1 mA and finally heldpotentiostatically at 0.0 volts for 24 hours. The difference inperformance of the two groups of cells in this test is shown in FIG. 9,where the capacity obtained in every cycle as a percent of the initialcapacity is plotted versus the cycle number. The cells with theconventional formation show large capacity fade, with the capacityfalling to approximately 52% after 160 deep discharge cycles. In sharpcontrast, the cells with the high voltage formation continue to showgreater than 95% of the initial capacity after 160 deep dischargecycles.

Rechargeable lithium-ion cells that utilize lithium titanate as anegative electrode active material may provide enhanced tolerance tooverdischarge conditions (e.g., conditions in which the lithium-ionbatteries are repeatedly cycled to near-zero volt conditions), asdescribed in U.S. patent application Ser. No. 10/979,040 filed Oct. 29,2004 (the entire disclosure of which is incorporated by referenceherein) and Example 2 below.

EXAMPLE 2

This example illustrates the overdischarge performance of lithium ionbatteries utilizing a negative electrode having Li₄Ti₅O₁₂ activematerial coated onto an aluminum current collector as compared to theperformance of lithium ion batteries in which a carbon active material(graphitized MCMB) is coated onto a copper current collector. In allcases, the electrodes were cycled against positive electrodes having aLiCoO₂ active material with a capacity of 5.53 mAh coated on an aluminumcurrent collector.

Negative electrodes were produced by mixing lithium titanate(commercially available from S

d Chemie Corporation) with a poly(vinylidine fluoride) binder, carbonblack and 1-methyl-2-pyrolidone (NMP) into a slurry and depositing themixture onto an aluminum foil current collector and drying on a heateddrum. The active weight percent of the dried coating was 89.25%. Threecoating deposition levels of the coating were used: 17.37, 18.78 and20.69 mg/cm². Based on the theoretical specific capacity of Li₄Ti₅O₁₂(155 mAh/g), the capacity of these electrodes was 4.76, 5.14 and 5.67mAh, respectively. Thus, when cycled against the positive electrodes,the cell balance (i.e., the ratio of the negative and positive electrodecapacities) was 0.85, 0.93 and 1.02, respectively.

After drying, the electrode coatings were calendared to a density ofabout 2.2 g/cm³ and cut into circular disks having an area of 1.98 cm².Lithium ion cells were produced by assembling these electrodes intotype-2032 coin cell hardware. A microporous polyolefin separator wasused to separate the negative and positive electrodes, and the cellswere activated by the addition of electrolyte consisting of 1 M LiPF₆ ina mixture of propylene carbonate, ethylene carbonate and diethylcarbonate.

Comparative cells were manufactured in identical fashion, with theexception of the fact that the negative electrodes utilized graphiteactive material (mesocarbon microbeads) coated on copper foil.

Cells were charged and discharged at a rate of 0.2 mA using an ARBINBT2000 battery cycler. For the first four cycles, the cells were cycledover a normal operating voltage range (3.0 to 1.8 V for the Li₄Ti₅O₁₂cells, 4.075 to 2.75 volts for the comparative cells). After completingfour cycles, the cells then underwent four overdischarge cycles, wherethey were discharged to 0 volts and charged back to their normal chargevoltage. The overdischarge took place as a sequence of steps atprogressively lower currents: 0.2 mA down to 1.8 volts, 0.05 mA to 1.0volts and 0.01 mA to 0 volts. Following the overdischarge cycles, thecells were then cycled over the original voltage range to measure therecovered capacity for the cells.

A graph showing the discharge capacity versus cycle number for each ofthe cells tested is shown in FIG. 12. Test data revealed a loss ofdischarge capacity over time after repeated overdischarge cycling forthe comparative cells (i.e., cells using carbon active material oncopper current collectors), while little or no loss of dischargecapacity was observed for cells utilizing Li₄Ti₅O₁₂ negative activematerial and an aluminum foil current collector.

Table 3 lists the cell discharge capacity of the cells during the fourthand ninth cycles of testing (i.e., the cycles immediately preceding andimmediately following the overdischarge cycles). For cells usingnegative electrodes with a Li₄Ti₅O₁₂ active material on an aluminumcurrent collector, there is little or no loss in capacity following theoverdischarge cycles. For the comparative cells (graphite activematerial on copper current collector), the average capacity loss wasobserved to be 84%.

Table 3 also lists the ratio of theoretical capacity of the negative andpositive electrodes for the Li₄Ti₅O₁₂ cells. This data indicates thatlittle or no capacity loss may be obtained regardless of whether thecells are negative limited (ratio of negative to positive electrodecapacity less than 1) or positive limited (ratio of negative to positiveelectrode capacity greater than 1).

TABLE 3 Negative/Positive Electrode Negative Capacity Ratio Cycle 4Cycle 9 Capacity Loss Group/Serial Active for Li₄Ti₅O₁₂ DischargeDischarge Due to Number Material Cells Capacity (mAh) Capacity (mAh)Overdischarge 17-CAL-01 Li₄Ti₅O₁₂ 0.86 4.12 4.13 −0.3% 17-CAL-02Li₄Ti₅O₁₂ 0.86 4.37 4.40 −0.6% 17-CAL-03 Li₄Ti₅O₁₂ 0.86 4.49 4.47 0.4%Average 4.33 4.33 −0.2% Standard Dev. 0.19 0.18 0.5% 19-CAL-01 Li₄Ti₅O₁₂0.93 4.81 4.83 −0.4% 19-CAL-02 Li₄Ti₅O₁₂ 0.93 4.36 4.40 −0.8% 19-CAL-03Li₄Ti₅O₁₂ 0.93 4.84 4.78 1.2% Average 4.67 4.67 0.0% Standard Dev. 0.270.24 1.1% 21-CAL-02 Li₄Ti₅O₁₂ 1.02 5.32 5.32 0.1% 21-CAL-03 Li₄Ti₅O₁₂1.02 4.37 4.23 3.2% Average 4.84 4.77 1.6% Standard Dev. 0.67 0.77 2.2%MCMB-02 Carbon n/a 4.99 1.68 66.4% MCMB-03 Carbon n/a 4.96 1.44 71.0%MCMB-04 Carbon n/a 4.88 0.14 97.1% MCMB-05 Carbon n/a 4.91 0.72 85.3%MCMB-06 Carbon n/a 4.88 0.00 100.0% Average 4.93 0.80 84.0% StandardDev. 0.05 0.75 15.1%

The batteries described herein may find utility in a variety ofapplications, including in implantable medical devices (IMDs). FIG. 13illustrates a schematic view of a system 400 (e.g., an implantablemedical device) implanted within a body or torso 432 of a patient 430.The system 400 includes a device 410 in the form of an implantablemedical device that for purposes of illustration is shown as adefibrillator configured to provide a therapeutic high voltage (e.g.,700 volt) treatment for the patient 430.

The device 410 includes a container or housing 414 that is hermeticallysealed and biologically inert according to an exemplary embodiment. Thecontainer may be made of a conductive material. One or more leads 416electrically connect the device 410 and to the patient's heart 420 via avein 422. Electrodes 417 are provided to sense cardiac activity and/orprovide an electrical potential to the heart 420. At least a portion ofthe leads 416 (e.g., an end portion of the leads shown as exposedelectrodes 417) may be provided adjacent or in contact with one or moreof a ventricle and an atrium of the heart 420.

The device 410 includes a battery 440 provided therein to provide powerfor the device 410. The size and capacity of the battery 440 may bechosen based on a number of factors, including the amount of chargerequired for a given patient's physical or medical characteristics, thesize or configuration of the device, and any of a variety of otherfactors. According to an exemplary embodiment, the battery is a 5 mAhbattery. According to another exemplary embodiment, the battery is a 300mAh battery. According to various other exemplary embodiments, thebattery may have a capacity of between approximately 1 and 1000 mAh.

According to other exemplary embodiments, more than one battery may beprovided to power the device 410. In such exemplary embodiments, thebatteries may have the same capacity or one or more of the batteries mayhave a higher or lower capacity than the other battery or batteries. Forexample, according to an exemplary embodiment, one of the batteries mayhave a capacity of approximately 500 mAh while another of the batteriesmay have a capacity of approximately 75 mAh.

According to an exemplary embodiment, the battery may be configured suchthat it may be charged and recharged using an inductive charging systemin which a primary or external coil is provided at an exterior surfaceof a portion of the body (either proximate or some distance away fromthe battery) and a secondary or internal coil is provided below the skinadjacent the primary coil.

According to another exemplary embodiment shown in FIG. 14, animplantable neurological stimulation device 500 (an implantable neurostimulator or INS) may include a battery 502 such as those describedabove with respect to the various exemplary embodiments. Examples ofsome neuro stimulation products and related components are shown anddescribed in a brochure titled “Implantable Neurostimulation Systems”available from Medtronic, Inc.

An INS generates one or more electrical stimulation signals that areused to influence the human nervous system or organs. Electricalcontacts carried on the distal end of a lead are placed at the desiredstimulation site such as the spine or brain and the proximal end of thelead is connected to the INS. The INS is then surgically implanted intoan individual such as into a subcutaneous pocket in the abdomen,pectoral region, or upper buttocks area. A clinician programs the INSwith a therapy using a programmer. The therapy configures parameters ofthe stimulation signal for the specific patient's therapy. An INS can beused to treat conditions such as pain, incontinence, movement disorderssuch as epilepsy and Parkinson's disease, and sleep apnea. Additionaltherapies appear promising to treat a variety of physiological,psychological, and emotional conditions. Before an INS is implanted todeliver a therapy, an external screener that replicates some or all ofthe INS functions is typically connected to the patient to evaluate theefficacy of the proposed therapy.

The INS 500 includes a lead extension 522 and a stimulation lead 524.The stimulation lead 524 is one or more insulated electrical conductorswith a connector 532 on the proximal end and electrical contacts (notshown) on the distal end. Some stimulation leads are designed to beinserted into a patient percutaneously, such as the Model 3487APisces-Quad® lead available from Medtronic, Inc. of Minneapolis Minn.,and stimulation some leads are designed to be surgically implanted, suchas the Model 3998 Specify® lead also available from Medtronic.

Although the lead connector 532 can be connected directly to the INS 500(e.g., at a point 536), typically the lead connector 532 is connected toa lead extension 522. The lead extension 522, such as a Model 7495available from Medtronic, is then connected to the INS 500.

Implantation of an INS 500 typically begins with implantation of atleast one stimulation lead 524, usually while the patient is under alocal anesthetic. The stimulation lead 524 can either be percutaneouslyor surgically implanted. Once the stimulation lead 524 has beenimplanted and positioned, the stimulation lead's 524 distal end istypically anchored into position to minimize movement of the stimulationlead 524 after implantation. The stimulation lead's 524 proximal end canbe configured to connect to a lead extension 522.

The INS 500 is programmed with a therapy and the therapy is oftenmodified to optimize the therapy for the patient (i.e., the INS may beprogrammed with a plurality of programs or therapies such that anappropriate therapy may be administered in a given situation).

A physician programmer and a patient programmer (not shown) may also beprovided to allow a physician or a patient to control the administrationof various therapies. A physician programmer, also known as a consoleprogrammer, uses telemetry to communicate with the implanted INS 500, soa clinician can program and manage a patient's therapy stored in the INS500, troubleshoot the patient's INS system, and/or collect data. Anexample of a physician programmer is a Model 7432 Console Programmeravailable from Medtronic. A patient programmer also uses telemetry tocommunicate with the INS 500, so the patient can manage some aspects ofher therapy as defined by the clinician. An example of a patientprogrammer is a Model 7434 Itrel® 3 EZ Patient Programmer available fromMedtronic.

According to an exemplary embodiment, a battery provided as part of theINS 500 may be configured such that it may be charged and rechargedusing an inductive charging system in which a primary or external coilis provided at an exterior surface of a portion of the body (eitherproximate or some distance away from the battery) and a secondary orinternal coil is provided below the skin adjacent the primary coil.

While the medical devices described herein (e.g., systems 400 and 500)are shown and described as a defibrillator and a neurologicalstimulation device, it should be appreciated that other types ofimplantable medical devices may be utilized according to other exemplaryembodiments, such as pacemakers, cardioverters, cardiac contractilitymodules, drug administering devices, diagnostic recorders, cochlearimplants, and the like for alleviating the adverse effects of varioushealth ailments.

It is also contemplated that the medical devices described herein may becharged or recharged when the medical device is implanted within apatient. That is, according to an exemplary embodiment, there is no needto disconnect or remove the medical device from the patient in order tocharge or recharge the medical device.

As utilized herein, the terms “approximately,” “about,” “substantially”,and similar terms are intended to have a broad meaning in harmony withthe common and accepted usage by those of ordinary skill in the art towhich the subject matter of this disclosure pertains. It should beunderstood by those of skill in the art who review this disclosure thatthese terms are intended to allow a description of certain featuresdescribed and claimed without restricting the scope of these features tothe precise numerical ranges provided. Accordingly, these terms shouldbe interpreted as indicating that insubstantial or inconsequentialmodifications or alterations of the subject matter described and claimedare considered to be within the scope of the invention as recited in theappended claims.

It should be noted that the term “exemplary” as used herein to describevarious embodiments is intended to indicate that such embodiments arepossible examples, representations, and/or illustrations of possibleembodiments (and such term is not intended to connote that suchembodiments are necessarily extraordinary or superlative examples).

It is important to note that the construction and arrangement of thebatteries and cells and the methods for forming such batteries as shownand described in the various exemplary embodiments is illustrative only.Although only a few embodiments have been described in detail in thisdisclosure, those skilled in the art who review this disclosure willreadily appreciate that many modifications are possible withoutmaterially departing from the novel teachings and advantages of thesubject matter recited in the claims. Accordingly, all suchmodifications are intended to be included within the scope of thepresent invention as defined in the appended claims. The order orsequence of any process or method steps may be varied or re-sequencedaccording to other exemplary embodiments. Other substitutions,modifications, changes, and omissions may be made in the design,operating conditions, and arrangement of the various exemplaryembodiments without departing from the scope of the present inventionsas expressed in the appended claims.

1. A rechargeable lithium-ion battery comprising: a positive electrodecomprising a first current collector and a first active material; anegative electrode comprising a second current collector and a secondactive material, the second active material comprising a lithiumtitanate material; and an electrolyte; wherein the positive electrodehas a first capacity and the negative electrode has a second capacity,the second capacity being less than the first capacity such that therechargeable lithium-ion battery is negative-limited.
 2. Therechargeable lithium-ion battery of claim 1, wherein the lithiumtitanate material comprises Li₄Ti₅O₁₂.
 3. The rechargeable lithium-ionbattery of claim 1, wherein the lithium titanate material is configuredto cycle lithium at a potential of greater than 0.2 volts versus alithium reference electrode.
 4. The rechargeable lithium-ion battery ofclaim 1, wherein the ratio of the second capacity to the first capacityis approximately 0.93.
 5. The rechargeable lithium-ion battery of claim1, wherein the ratio of the second capacity to the first capacity isapproximately 0.85.
 6. The rechargeable lithium-ion battery of claim 1,wherein the ratio of the second capacity to the first capacity is lessthan approximately 0.90.
 7. The rechargeable lithium-ion battery ofclaim 6, wherein the ratio of the second capacity to the first capacityis greater than approximately 0.70.
 8. The rechargeable lithium-ionbattery of claim 7, wherein the ratio of the second capacity to thefirst capacity is greater than approximately 0.80.
 9. The rechargeablelithium-ion battery of claim 8, wherein the ratio of the second capacityto the first capacity is approximately 0.84.
 10. The rechargeablelithium-ion battery of claim 1, wherein the ratio of the second capacityto the first capacity is approximately 0.73.
 11. The rechargeablelithium-ion battery of claim 1, wherein the first active materialcomprises LiCoO₂.
 12. The rechargeable lithium-ion battery of claim 1,wherein the electrolyte consists essentially of ethylene carbonate,ethylmethyl carbonate, and a salt of LiPF₆.
 13. The rechargeablelithium-ion battery of claim 1, wherein the electrolyte does not includea molten salt.
 14. The rechargeable lithium-ion battery of claim 1,wherein the electrolyte does not include additives that are intended toprevent degradation due to oxidation of the positive electrode.
 15. Therechargeable lithium-ion battery of claim 14, wherein the electrolytedoes not include biphenyl.
 16. The rechargeable lithium-ion battery ofclaim 1, wherein the battery is configured for use in a medical device.17. The rechargeable lithium-ion battery of claim 1, wherein thepositive electrode and the negative electrode are provided as woundelectrodes.
 18. A rechargeable lithium-ion battery comprising: apositive electrode comprising a first current collector and a firstactive material; a negative electrode comprising a second currentcollector and a second active material, the second active materialcomprising a material that cycles lithium at a potential of greater than0.2 volts versus a lithium reference electrode; and an electrolytecomprising ethylene carbonate and free of molten salt; wherein thepositive electrode has a first capacity and the negative electrode has asecond capacity, the second capacity being less than the first capacity;whereby the rechargeable lithium-ion battery exhibits improvedresistance to capacity fade after repeated cycling as compared tobatteries having balanced electrode capacities.
 19. The rechargeablelithium-ion battery of claim 18, wherein the second active material is alithium titanate material.
 20. The rechargeable lithium-ion battery ofclaim 19, wherein the lithium titanate material comprises Li₄Ti₅O₁₂. 21.The rechargeable lithium-ion battery of claim 20, wherein the firstactive material comprises LiCoO₂.
 22. The rechargeable lithium-ionbattery of claim 18, wherein the electrolyte further comprisesethylmethyl carbonate, and a salt of LiPF₆.
 23. The rechargeablelithium-ion battery of claim 18, wherein the ratio of the secondcapacity to the first capacity is less than approximately 0.90.
 24. Therechargeable lithium-ion battery of claim 23, wherein the ratio of thesecond capacity to the first capacity is greater than approximately0.70.
 25. The rechargeable lithium-ion battery of claim 24, wherein theratio of the second capacity to the first capacity is greater thanapproximately 0.80.
 26. The rechargeable lithium-ion battery of claim18, wherein the ratio of the second capacity to the first capacity isapproximately 0.84.
 27. The rechargeable lithium-ion battery of claim18, wherein the ratio of the second capacity to the first capacity isapproximately 0.73.
 28. The rechargeable lithium-ion battery of claim18, wherein the battery is configured for use in a medical device. 29.The rechargeable lithium-ion battery of claim 18, wherein the positiveelectrode and the negative electrode are provided as wound electrodes.30. A rechargeable lithium-ion battery comprising: a positive electrode;a negative electrode comprising an active material that cycles lithiumat a potential of greater than 0.2 volts versus a lithium referenceelectrode; and an electrolyte that is free of molten salt; wherein therechargeable lithium-ion battery has a negative-limited cell balancesuch that the positive electrode has greater capacity than the negativeelectrode; whereby the rechargeable lithium-ion battery exhibitsimproved resistance to capacity fade after repeated cycling as comparedto batteries having balanced electrode capacities.
 31. The rechargeablelithium-ion battery of claim 30, wherein the active material is alithium titanate material.
 32. The rechargeable lithium-ion battery ofclaim 31, wherein the lithium titanate material comprises Li₄Ti₅O₁₂. 33.The rechargeable lithium-ion battery of claim 32, wherein the positiveelectrode comprises an active material comprising LiCoO₂.
 34. Therechargeable lithium-ion battery of claim 30, wherein the molten saltfree electrolyte consists essentially of ethylene carbonate, ethylmethylcarbonate, and a salt of LiPF₆.
 35. The rechargeable lithium-ion batteryof claim 30, wherein the cell balance is less than approximately 0.90.36. The rechargeable lithium-ion battery of claim 35, wherein the cellbalance is greater than approximately 0.70.
 37. The rechargeablelithium-ion battery of claim 36, wherein the cell balance is greaterthan approximately 0.80.
 38. The rechargeable lithium-ion battery ofclaim 30, wherein the cell balance is approximately 0.84.
 39. Therechargeable lithium-ion battery of claim 30, wherein the cell balanceis approximately 0.73.
 40. The rechargeable lithium-ion battery of claim30, wherein the battery is configured for use in a medical device. 41.The rechargeable lithium-ion battery of claim 30, wherein the positiveelectrode and the negative electrode are provided as wound electrodes.